Projection objective for microlithography

ABSTRACT

A projection objective for microlithography for imaging a pattern arranged in the object plane of the projection objective into the image plane of the projection objective has at least one polarization splitter device that is operated only once in transmission or reflection. By using this device, polarization-dependent differences in the intensity and response of the light passing through the objective, which lead to a worsening of the imaging quality of the projection objective, can largely be avoided.

This is a continuation of application Ser. No. 10/919,376 filed Aug. 17,2004. The entire disclosure of the prior application, application Ser.No. 10/919,376, which is incorporated into this application byreference, which is based on German patent Application No. 10338983.0filed on Aug. 20, 2003, which is incorporated into this application byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a projection objective for microlithography forimaging a pattern arranged in an object plane of the projectionobjective into an image plane of the projection objective.

2. Description of the Related Art

Projection objectives of this type are used in projection exposuresystems for the production of semiconductor components and other finelystructured components, in particular in wafer scanners and wafersteppers. They are used to project patterns of photo masks or graduatedplates, which will also be designated masks or reticles in the followingtext, onto an object coated with a light-sensitive layer at the maximumresolution and on a reducing scale.

In this case, in order to produce finer and finer structures, it isnecessary firstly to increase the image-side numerical aperture (NA) ofthe projection objective and secondly to use shorter and shorterwavelengths, preferably ultraviolet light with wavelengths of less thanabout 260 nm.

In this wavelength range, only a few adequately transparent materialsare available for the production of optical components, in particularsynthetic quartz glass and fluoride crystals such as calcium fluoride.By means of intrinsic and/or voltage-induced birefringence, when lightpasses through these materials, a difference in the path between lighthaving a first component of the electric field strength vector and lighthaving a second component of the electric field strength vectorperpendicular to this first component can occur.

Since the Abbé constants of the materials available for this wavelengthrange lie relatively close to one another, it is difficult to providepurely refractive systems with adequate correction of color errors.Therefore, for high-resolution projection objectives use ispredominantly made of catadioptric systems in which, apart fromrefractive elements, reflective elements, for example concave mirrorsand deflection mirrors, are also used. During reflection at such anelement, the light is generally influenced as a function ofpolarization, specifically in such a way that a difference in responsecan occur between light having a first component of the electrical fieldstrength vector, which oscillates perpendicular to the plane ofincidence (s-polarized light) and light having a second component, whichoscillates parallel to the plane of incidence (p-polarized light).

The two facts outlined above can contribute to the optical path lengthtraced in the projection objective by the light having a first componentof the electrical field strength vector differing from that traced bythe light having a second component of the electrical field strengthfactor perpendicular to the first component. In the image plane, theresult is the production of two mutually offset partial images (doubleimages), which results in a worsening of the image contrast.

In catadioptric systems, it has additionally been observed that, undercertain imaging conditions, various structure directions contained inthe pattern to be imaged are imaged with different contrast, so that, inthe photoresist, different line widths occur from the various structuredirections. This can be attributed, inter alia, to the fact that, at areflective optical component, such as is normally used in catadioptricprojection objectives for microlithography, light polarizedperpendicular to the plane of incidence is reflected more strongly thanlight polarized parallel to the plane of incidence, that is to say theintensity ratio of s-polarized to p-polarized light at the exit from theprojection objective differs from the intensity ratio at the entry.Since the deflection mirrors used in catadioptric systems are normallyoperated with large angles of incidence, at which the difference in thereflectance between s-polarized and p-polarized light is particularlyhigh, considerable differences can occur in the intensity ratio ofs-polarized to p-polarized light at the entry and exit from theprojection objective.

Patent publication DE 198 51 749 A1 discloses a catadioptric projectionobjective in which effects dependent on polarization, which arise as aresult of different reflection as a function of the direction ofpolarization and therefore produce differences in response anddisplacement of the position of the beam of rays producing the image onthe wafer, are compensated for by matching dielectric reflex layers orby means of additional non-coplanar deflections. The disadvantage withthe last-named solution is that it requires considerable expenditure onconstruction.

Patent application DE 102 40 598.0 from the applicant, which is not aprior publication, discloses an optical imaging system having a firstand second reflection mirror, in which a ratio R_(sp) between thereflectance R_(s) of a deflection mirror for s-polarized light and thereflectance R_(p) of the deflection mirror for p-polarized light from arange of angles of incidence covering the associated tilt angle isgreater than one in the case of one of the deflection mirrors and lessthan one in the case of the other deflection mirror. Such an apparatuscan be used for the purpose of compensating for thepolarization-changing action of one deflection mirror with the aid ofthe second deflection mirror.

SUMMARY OF THE INVENTION

It is one object of the invention to provide a projection objective formicrolithography in which negative influences on the imaging quality,which can be produced by polarization-dependent effects as light passesthrough this projection objective, are reduced substantially.

As a solution to this and other objects, the invention, according to oneformulation, provides a projection objective for microlithography forimaging a pattern arranged in an object plane of the projectionobjective into an image plane of the projection objective comprising atleast one polarization splitter device that is operated only once intransmission or reflection between object plane and image plane.

Advantageous developments are specified in the dependent claims. Thewording of all the claims is incorporated in the content of thedescription by reference.

A projection objective according to the invention has, at least onepoint, a polarization splitter device which largely or substantiallycompletely discards one direction of polarization of the light whichpasses through the projection objective as far as the polarizationsplitter device. A difference in response arising in the light pathbefore this polarization splitter device between two mutuallyperpendicular directions of polarization therefore no longer plays anypart in the imaging properties of the projection objective. In the lightpath after the polarization splitter device, by using only onepolarization component, a difference in response can no longer occur,provided that polarization-changing elements are no longer used there.The intensity difference explained at the beginning between s-polarizedand p-polarized light following reflection at reflective opticalcomponents which, for example, are used for beam folding in dioptric orin catadioptric systems, can likewise be avoided by the use of apolarization splitter device according to the invention. Thepolarization splitter device is used only once in transmission orreflection, as a result of which light losses are limited and undesireddisruption of the image by the polarization splitting can be minimized.

Since the imaging quality of the projection objective is improvedsignificantly by the invention, an intensity loss in the light, possiblyarising as a result of filtering of a polarization component, plays asubordinate part in many cases. This can be limited by suitablemeasures, for example by the selection of a suitable illumination mode.

Wavefront aberrations, such as can occur in a projection objective formicrolithography, may normally be corrected effectively only for onedirection of polarization; if it is wished to optimize the projectionobjective for a second direction of polarization perpendicular to afirst direction of polarization, the result of the correction which hasbeen made for the first direction of polarization is necessarily madeworse. The invention permits effective correction of wavefrontaberrations since only one direction of polarization contributes to theproduction of the image, for which the projection objective can becorrected effectively by means of matching or aspherizing lens elements.

If the polarization splitter device is designed as a transparent opticalelement provided with at least one polarization splitter structure, thenthe polarization splitter structure can be applied, for example, to thefront and/or rear of this optical element. In a preferred embodiment, atleast one polarization splitter structure is applied to an opticalelement already present in the projection objective. In this case, anoptical element can be constructed in one piece or many parts.

A polarization splitter structure can be formed in various ways, forexample as a multilayer system in which dielectric layers with a highand low refractive index alternate. Such layer systems are customarilyoperated at angles of incidence in the vicinity of the Brewster angle,that is to say typically at an angle of incidence of about 45°. Suitabledielectric polarization splitter multilayer systems of this kind aredescribed, for example, in U.S. Pat. No. 6,480,330 B1.

In particular for very small angles of incidence, that is to say if thelight striking the polarization splitter structure strikes itsubstantially perpendicularly, it is possible to use polarizationsplitter structures whose structure sizes lie below the wavelength ofthe light used. Such structures act like birefringent material on thelight passing through, on account of their form or structure, and cantherefore be used for polarization splitting. Polarization splitterswhich use this effect of “form birefringence” are described, forexample, in U.S. Pat. No. 6,384,974 B1.

It is possible to form the polarization splitter device as aplane-parallel plate, which is arranged in such a way that it istransilluminated substantially perpendicularly by the light passingthrough the projection objective, so that said plate, apart from thefiltering of a direction of polarization, which, for example, can beachieved by applying a polarization-selective diffractive structure tothe plate, has no additional influence on the course of the light path.

It is also possible to introduce a plane-parallel plate obliquely intothe beam path, that is to say inclined at a predefined angle withrespect to a plane perpendicular to the optical axis. When light passesthrough this plate, even in the case of an uncoated plate, there is moreintense attenuation during transmission of the light polarizedperpendicular to the plane of incidence as compared with light polarizedparallel to the plane of incidence, so that at least partial filteringof s-polarized light is achieved without the application ofpolarization-changing layers. In this case, the plane-parallel plateshould be introduced into the beam path at a point at which the incidentbundle of light consists of substantially parallel partial rays. In theevent of non-parallel incidence of light, a variation in thepolarization state occurs for different rays in the bundle. In thiscase, the plane-parallel plate can be matched in terms of itspolarization-changing action to other optical elements of the projectionobjective, for example to a folding mirror placed obliquely, in such away that a desired, in particular an overall polarization-maintaining,influence on the polarization occurs. The offset of the light beamoccurring when a plane-parallel plate introduced obliquely into the beampath is used, and any imaging errors which may possibly occur which arenot rotationally symmetrical with respect to the optical axis, can becompensated for again by means of a further plate with similardimensioning which, with respect to a plane perpendicular to the opticalaxis, is inclined oppositely to the first plate.

If the polarization splitter device is arranged as the last opticalelement of the objective in the vicinity of the image plane, then thereare no longer any polarization-changing elements in the beam path aftersaid polarization splitter device. The polarization state present at theexit from the polarization splitter device is therefore availableunchanged for the imaging.

In a preferred embodiment, the polarization splitter device forms thepossibly interchangeable terminating plate of the objective, so that noadditional optical element has to be introduced into the objective. Ifthe terminating plate reflects s-polarized light and lets p-polarizedlight through then, in the case of annular illumination andappropriately convergent, annular incidence of light, this leads toprimarily radially polarized light being available for the production ofan image, which effects good injection of the light into the resist. Thelight loss can be kept low in this embodiment if the illumination islikewise radially polarized.

In one embodiment, there is in the projection objective at least onedeflection mirror, usually for the purpose of beam folding, which isformed as a polarization splitter device. If the polarization splittingis effected by at least one polarization splitter structure beingapplied to the mirror surface for this purpose, and being formed as adielectric multilayer system in order to reflect essentially s-polarizedlight, then such a multilayer system can be produced particularly easilysince this effect, which is particularly highly pronounced at highangles of incidence, that s-polarized light is reflected more intenselyat a reflective surface than p-polarized light, merely needs to beamplified.

In one embodiment, the projection objective constitutes a catadioptricobjective having a concave mirror and a beam deflection device. In thiscase, the beam deflection device can have a first mirror surface fordeflecting the radiation coming from the object plane to the concavemirror and a second mirror surface for deflecting the radiationreflected from the concave mirror in the direction of the image plane.In this embodiment, one or both mirror surfaces can be formed as apolarization splitter device. Other folding geometries are likewisepossible, for example those in which the light coming from the objectplane strikes the concave mirror first and only then one or moredeflection mirrors. In these variants, it is advantageous that noadditional structural elements have to be used, since the polarizationsplitter device is an optical element already present in the objective.

If, when two (or more) deflection mirrors are used, the first mirrorsurface is chosen to be a polarization splitter device and the latter isformed in such a way that essentially p-polarized light is reflectedfrom the mirror surface, then it is possible for a non-negligible partof the light intensity to be lost at the second mirror surface, but thefollowing lenses can effectively be rendered nonreflective. A mirrorsurface at which p-polarized light is reflected more strongly thans-polarized light is described, for example, in DE 102 40 598 from theapplicant, already cited above, whose disclosure content is incorporatedin the content of this description by reference. If essentiallys-polarized light is reflected from the first mirror surface, a greaterpart of this light will be reflected at the second mirror surface, butunder certain circumstances it is more difficult to effectively renderfollowing lenses nonreflective.

If the second mirror surface is chosen to be a polarization splitterdevice, then the dioptric part of the objective follows directly in thecourse of the beam. Only the following lenses are therefore still ableto change the polarization state.

In the beam path after the polarization splitter device there islinearly polarized light. However, if the photoresist is exposed in theimage plane of the projection objective, it is frequently not intendedfor linearly polarized light to be used, since this can lead to contrastdifferences which depend on the structure direction. It is thereforeexpedient to convert the linear polarization of the light intocircularly polarized light before leaving the objective, by means of asuitable delay element, for example a λ/4 plate.

In many variants of the invention, the polarization splitter device usedis an optical element which is already present in the objective, as aresult of which the expenditure on the structure needed for theimplementation of the invention can be kept low, which contributes tosaving costs. The use of the optical element as a polarization splitterdevice or modification of a conventional transparent or reflectivecomponent to form a polarization splitter device is made possible by thefact that at least one polarization splitter structure is applied tosaid element, for example a dielectric multilayer system, a structurewith structure-induced birefringence (form birefringence) or adiffractive structure. As a result of the low expenditure on thestructure, it is in this case possibly also possible to equip projectionobjectives that have already been finished with a polarization splitterdevice according to the invention.

The above and further features emerge from the description and thedrawings, as well as from the claims, it being possible for theindividual features in each case to be implemented on their own or in aplurality in the form of subcombinations in embodiments of the inventionand in other fields and to represent embodiments which are advantageousand capable of protection on their own.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a microlithography projectionexposure system formed as a wafer scanner, which comprises acatadioptric projection objective with geometric beam splittingaccording to one embodiment of the invention;

FIG. 2 is a schematic view of an embodiment of a projection objectiveaccording to the invention having a plane-parallel plate acting as apolarization splitter device in the vicinity of the light entry;

FIG. 3 is a schematic detailed view of the dioptric terminating part ofan embodiment of a projection objective according to the inventionhaving a terminating plate acting as a polarization splitter device;

FIG. 4 is a schematic detailed view of the catadioptric objective partof an embodiment of a projection objective according to the inventionhaving a first mirror surface acting as a polarization splitter device;and

FIG. 5 is a schematic illustration of the catadioptric objective part ofan embodiment of a projection objective according to the inventionhaving a second mirror surface acting as a polarization splitter device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 a microlithography projection exposure system in the form of awafer scanner 1, which is provided for the production of highlyintegrated semiconductor components, is shown schematically. Theprojection exposure system comprises, as light source, an excimer laser2, which emits ultraviolet light with a working wavelength of 157 nmwhich, in other embodiments, can also be above this, for example at 193nm or 248 nm, or below this. An illumination system 4 connecteddownstream produces a large, sharply delimited and homogeneouslyilluminated image field, which is matched to the telecentricrequirements of the projection objective 5 connected downstream. Theillumination system has devices for selection of the illumination modeand, for example, can be changed over between conventional illuminationwith a variable coherence level, annular field illumination and dipoleor quadrupole illumination. Arranged after the illumination system is adevice 6 for holding and manipulating a mask 7, such that the mask liesin the object plane 8 of the projection objective and can be moved inthis plane by means of a scanning drive in a direction of travel 9 (ydirection) for scanner operation.

After the mask plane 8 there follows the projection objective 5, whichacts as a reduction objective and projects an image of a patternarranged on the mask on a reduced scale, for example on a scale of 1:4or 1:5, onto a wafer 10 which is covered with a photoresist layer andwhich is arranged in the image plane 11 of the reduction objective.Other reduction scales, for example more intense reductions of up to1:20 or 1:200, are possible. The wafer 10 is held by a device 12 whichcomprises a scanner drive in order to move the wafer synchronously withthe reticle 7 and parallel to the latter. All the systems are controlledby a control unit 13.

The catadioptric projection objective 5 operates with geometric beamsplitting and, between its objective plane (mask plane 8) and its imageplane (wafer plane 11), has a catadioptric objective part 15 having afirst deflection mirror 19 and a concave mirror 16, the flat deflectionmirror 19 being tilted with respect to the optical axis 23 of theprojection objective such that the radiation coming from the objectplane is deflected by the deflection mirror 19 in the direction of theconcave mirror 16. In addition to this mirror 19 needed for the functionof the projection objective, a second, flat deflection mirror 20 isprovided, which is tilted with respect to the optical axis in such a waythat the radiation reflected from the concave mirror 16 is deflected bythe deflection mirror 20 in the direction of the image plane 11 to thelenses of the following dioptric objective part 18. The mirror surfaces19, 20, which are perpendicular to each other and flat, are provided ona beam deflection device 17 formed as a mirror prism and have paralleltilt axes perpendicular to the optical axis 23.

At the end of the dioptric objective part 18 that faces the image plane,a terminating plate 40 is fitted as the last optical element, to whoselight entry side a plurality of flat layers 42 which act overall in apolarization-selective manner are applied. With regard to thefunctioning of the polarization-selective terminating plate, referenceshould be made to the description in connection with FIG. 3.

A view of an embodiment of a projection objective according to theinvention is shown schematically in FIG. 2. The light which enters theprojection objective is made available by the illumination system inunpolarized form in the case described here, but other polarizationstates, for example radial, circular or linear polarization states, ofthe entering light are also possible.

In front of the first deflection mirror 19 of the projection objective,a plane-parallel plate 30 made of CaF₂ or synthetic quartz glass isfitted, which lies in a plane perpendicular to the optical axis, so thatit is transilluminated essentially perpendicularly by the light passingthrough the projection objective. This leads to the light path in theprojection objective not being disrupted by the plate. Applied to theentry surface of this plate 30 is a polarization splitter structure 31in the form of a diffraction grating with a structure width which liesbelow the wavelength of the light that passes through the projectionobjective. Such a grating, in which the diffractive structures point ina predefined direction, acts like a birefringent material, so that sucha grating can be used as a polarization splitter. The grating 31transmits essentially light having a first component of the electricfield strength vector, whereas it essentially reflects light having asecond component perpendicular to the first component of the electricfield strength vector. The plate 30 covered with the diffraction grating31 forms a polarization splitter device.

The manufacture of such gratings is described, for example, in thearticle “Electron-Beam Lithography for Fabricating Sub-WavelengthGrating on Quartz” by Shr-Jia Shiu and Hang-Peng Shieh (National ChiaoTung University Hsinchu, Taiwan). For a more detailed description of thefunctioning of polarization splitter structures with diffractionratings, which comprise a plurality of periodically arranged layers,reference should be made to the article “Polarizing Beam SplittersConstructed of Form Birefringent Multilayer Gratings” by R.-C. Tyan,P.-C. Sun and Y. Fainman in: SPIE, Vol. 2689, 82-89. A polarizationsplitter structure which has two diffraction gratings with structurewidths below the light wavelength is described in US 2002/0164105 A1.

In the present embodiment, the linear polarization produced by the plate30 and the diffraction grating 31 is chosen such that the field strengthvector of the electric field is perpendicular to the plane of incidenceof the light on the first diffraction mirror 19. The light coming fromthe plate 30 is reflected by the first deflection mirror 19, it provingto be beneficial that this is s-polarized with respect to the mirrorplane, since s-polarized light is reflected by the mirror surface moreintensely than p-polarized light, so that only a little light intensityis lost at the first deflection mirror 19.

The light reflected from the first deflection mirror 19 strikes theconcave mirror 16, is reflected from the latter and then strikes thesecond deflection mirror 20, from which it is reflected in the directionof the image plane 11. In the reflection at the second deflection mirror20, it again proves to be beneficial that the light is s-polarized withrespect to the plane of incidence, so that only a little light intensityis lost at the second mirror 20.

The light passes further through the dioptric part 18 of the objectiveand, at the end of the latter, strikes a λ/4 plate 31, which has thepurpose of converting the linearly polarized light coming from thedioptric part of the projection objective into circularly polarizedlight, so that no direction-dependent contrast differences are producedon the wafer 10.

When this embodiment of the invention is used, polarization-dependent,negative effects on the imaging quality, in particular the production ofdouble images and of contrast differences that are dependent on thestructure direction, are largely avoided.

A detailed view of the dioptric terminating part 18 of a projectionobjective is shown schematically in FIG. 3. The light passing throughthis dioptric part of the objective strikes a multilayer system 42 thatis applied to the entry surface 14 of a terminating plate 40, which actsin a polarization-selective manner in such a way that said system allowsonly light that is p-polarized in relation to the plane of incidence ofthe terminating plate to pass through. In a preferred exposure process,the light at the end of the dioptric part has an annular distribution asa result of the use of an annular illumination mode in the illuminationsystem. In this part of the objective close to the image, the light hasa highly convergent beam path. The terminating plate 40 is fitted in aplane lying perpendicular to the optical axis 23, which, on account ofthe annular incidence of the light and the convergent beam path, isstruck by the incident light in such a way that the planes of incidenceof the incident light beams are arranged rotationally symmetrically withrespect to the optical axis. Since the polarizing layers 42 let throughonly light polarized parallel to the planes of incidence of the lightbeams, and the planes of incidence are rotationally symmetrical withrespect to the optical axis, after these layers, or after theterminating plate, radially polarized light is available for theproduction of the image on the wafer 10. This effects good injection ofthe light into the resist, since only a small proportion of the lightintensity is lost by reflection during the injection of radiallypolarized light.

FIG. 4 shows a schematic detailed view of the catadioptric objective 15of an embodiment of a projection objective according to the invention,having a first deflection mirror 19 acting as a polarization splitterdevice, a concave mirror 16 and a second deflection mirror 20. Appliedto the first deflection mirror 19, acting as a polarization splitterdevice, is a dielectric multilayer system 44, which filters theunpolarized light coming from the illumination system into polarizedlight perpendicular to the plane of incidence of the first deflectionmirror 19. It is possible to produce highly effective dielectricmultilayer systems 44, so that the light which passes through theprojection objective is present essentially in linearly polarized formafter passing through such a multilayer system 44. The multilayer system44 comprises a plurality of individual layers arranged one aboveanother, which consist in an alternating sequence of materials with ahigh and low refractive index for the wavelength ranges used. As aresult of the interference between the light beams reflected at theseindividual layers, light having a first direction of polarization ispreferably reflected, whereas light having a second componentperpendicular thereto is essentially discarded.

The light reflected from the first deflection mirror strikes the concavemirror 16, is reflected by the latter and reaches the second deflectionmirror 20, from which the light path runs onwards into the dioptric part18 of the objective. Since there is only s-polarized light upstream ofthe second deflection mirror 20, the light loss during the reflection atthe second deflection mirror 20 can be kept low. In this embodiment,too, further changes in the polarization state can be effected in thefollowing beam path, in order to obtain, for example, radial ortangential polarization in the image plane.

FIG. 5 shows a schematic detailed view of the catadioptric objectivepart 15 of an embodiment of a projection objective according to theinvention having a first deflection mirror 19, a concave mirror 16 and asecond deflection mirror 20, which is formed as a polarization beamsplitter. The unpolarized light coming from the illumination unit fallson the first deflection mirror 19, is reflected from the latter towardsthe concave mirror 16, which throws the light onto the second deflectionmirror 20. Applied to the second mirror surface 20, acting as apolarization splitter device, are a plurality of dielectric layers 45,which filter the unpolarized light coming from the concave mirror intolight polarized parallel to the plane of incidence of the second mirrorsurface. As a result, the following lenses in the dioptric part 18 ofthe projection objective can be effectively rendered nonreflective. Inthis embodiment, too, further changes in the direction of polarizationcan be performed in the following beam path, in order to obtain, forexample, radial or tangential polarization in the image plane.

In the embodiments shown in FIGS. 3, 4 and 5, the polarization splitterdevice is formed by applying a polarization splitter structure to anoptical element already present in the projection objective, whereas,for the embodiment shown in FIG. 2, an additional optical element in theform of a plane-parallel plate was introduced into the beam path.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. It is sought, therefore, to cover allchanges and modifications as fall within the spirit and scope of theinvention, as defined by the appended claims, and equivalents thereof.

1. A microlithography projection objective for imaging a patternarranged in an object plane of the projection objective onto an imageplane of the projection objective, the projection objective comprising:a transparent optical element arranged in a vicinity of the image plane;the transparent optical element comprising a polarization splitterstructure operated only once in transmission as rays of a light beamtravel from the object plane to the image plane; wherein thepolarization splitter structure transmits substantially only onedirection of polarization.
 2. The projection objective according toclaim 1, wherein the polarization splitter structure is disposed in alocation at which light rays incident thereon are convergent and planesof incidence of the convergent light rays are rotationally symmetric toan optical axis of the projection objective, and wherein substantiallyall the light transmitted by the polarization splitter structure has thesame direction of polarization with respect to the plane of incidence ofthe light on the polarization splitter structure, such that thepolarization directions of light transmitted by the polarizationsplitter structure are substantially rotationally symmetric with respectto the optical axis.
 3. The projection objective according to claim 2,wherein the polarization splitter structure transmits only light rayspolarized parallel to their planes of incidence, such that radiallypolarized light is incident on the image plane.
 4. The projectionobjective according to claim 1, wherein the transparent optical elementis the final optical element of the projection objective in the beamdirection.
 5. The projection objective according to claim 4, wherein thetransparent optical element is a plane-parallel plate.
 6. The projectionobjective according to claim 1, wherein the transparent optical elementis a terminating plate of the projection objective immediately adjacentto the image plane, and wherein the projection objective furthercomprises a dielectric multi-layer system disposed on an entry surfaceof the terminating plate, the multilayer system forming the polarizationsplitter structure.
 7. The projection objective according to claim 1,wherein the polarization splitter structure comprises apolarization-selective dielectric multilayer system.
 8. The projectionobjective according to claim 1, wherein the transparent optical elementis interchangeable.
 9. The projection objective according to claim 1,wherein the projection objective is configured to generate tangentialpolarization in the image plane.
 10. The projection objective accordingto claim 1, wherein the projection objective is a catadioptricprojection objective comprising at least one concave mirror.
 11. Theprojection objective according to claim 1, further comprising: acatadioptric first objective part comprising a concave mirror and a beamdeflection device, and, disposed behind the beam deflection device, adioptric second objective part.
 12. The projection objective accordingto claim 11, wherein the beam deflection device comprises: a firstmirror surface for deflecting radiation from the object plane toward theconcave mirror, and a second mirror surface for deflecting radiationfrom the concave mirror toward the second objective part.
 13. Amicrolithography projection objective for imaging a pattern arranged inan object plane of the projection objective into an image plane of theprojection objective, the projection objective comprising: at least oneoptical element comprising a polarization splitter structure throughwhich light passes only once between the object plane and the imageplane, and which transmits only one direction of polarization; whereinthe projection objective is configured to generate tangentialpolarization in the image plane.
 14. The projection objective accordingto claim 13, wherein the projection objective is a catadioptricprojection objective comprising at least one concave mirror.
 15. Theprojection objective according to claim 13, further comprising: acatadioptric first objective part comprising a concave mirror and a beamdeflection device, and, disposed behind the beam deflection device, adioptric second objective part.
 16. The projection objective accordingto claim 15, wherein the beam deflection device comprises a first mirrorsurface for deflecting radiation from the object plane toward theconcave mirror, and a second mirror surface for deflecting radiationfrom the concave mirror toward the second objective part.